1. Field of Invention
This invention relates to deformable mirrors in general, and more particularly to electrodisplacive transducers for selectively deforming a mirror's reflecting surface and a method of manufacturing transducers for deformable mirrors.
2. Description of the Prior Art
The use of multilayer electrodisplacive actuators in deformable mirrors is well know in the prior art. See, for example, U.S. Pat. No. 4,657,358 which issued Apr. 14, 1987 and which discloses the use of multiple actuators for selectively imparting deformations into the reflecting surface of a deformable mirror. The actuators are each composed of multiple layers of electrodisplacive material, for example lead magnesium niobate, which are made to elongate by the application of an electrical field between each layer of the electrodisplacive material.
Those skilled in the art of deformable mirror construction are aware that the operation of a deformable mirror relies on the use of actuators which can be selectively operated to impart a precise force against the mirror's reflecting surface to impart precisely controlled deformations across the reflecting surface. For deformable mirrors utilizing large numbers of actuators to effect precise control of the mirror's reflecting surface, it is important that each actuator produce the same deformation in the mirror's reflecting surface upon the application of electrical signals of equal magnitude. Failure to produce uniform deformations among different actuators results in errors being introduced into the surface configuration of the reflecting surface. While such nonlinearities between actuators in a mirror may be overcome by adjusting the bias applied to each actuator to tailor the actuators response curve, i.e., the amount of deformation provided for a given electrical signal, such adjustments are time consuming and frequently difficult to achieve due to the requirement to individually test and balance each of many actuators in the deformable mirror.
One approach for overcoming inconsistencies in response between actuators destined for use in a deformable mirror is to test each actuator with a common electrical signal prior to its placement in a deformable mirror. During the testing cycle, those actuators which exhibit a common response, i.e., which exhibit the same elongation upon the application of a common electrical signal, are chosen for use in a particular deformable mirror. Other actuators which exhibit response curves differing from the chosen units are rejected.
One method for manufacturing actuators for use in deformable mirrors involves the use of multiple layers of electrodisplacive material which are stacked upon each other to provide increased actuator stroke, i.e., elongation in the direction parallel to the applied electric field. The number of layers of electrodisplacive material in each actuator is determined by the stroke that each actuator is to produce to deform the mirror's reflecting surface. The amount of stroke produced by an actuator, is directly proportional to the number of layers of electrodisplacive material. To achieve significant mechanical displacement (i.e., stroke) along the axis of the layers of actuator material, each actuator must be composed of many layers of electrodisplacive material. It is not uncommon to utilize actuators of 60 or more layers of electrodisplacive material to permit a stroke of 5 microns to be produced utilizing an electrical signal of approximately 200 volts. Layers of electrodisplacive material are connected mechanically in series by bonding each layer of actuator material to adjacent layers above and below, and electrically in parallel. The mechanical connection between adjacent layers permits each layer's stroke to be added to the stroke of the other layers in the actuator, thereby yielding a larger stroke than that achievable with a single layer of electrodisplacive material.
The layers of electrodisplacive material are manufactured by tape casting a slurry of electrodisplacive material, for example lead magnesium niobate in a binder, to produce sheets of electrodisplacive material having a specified thickness. The individual layers of electrodisplacive material are then coated on one side with a platinum ink in a pattern to produce an electrode of predetermined dimensions in the completed actuators. The layers of electrodisplacive material are then stacked upon each other and are pressed into a block which is then fired while held under pressure in an oven to produce a ceramic structure. The block is then diced or core drilled to produce multilayer actuators. The electrode patterns are then externally connected in alternating layers to permit an electric field to be generated between each layer of electrodisplacive material by the application of an electrical signal to the layers of electrodes. The foregoing manufacturing process is labor-intensive, requiring considerable attention by skilled technicians to assure the manufacture of actuators without defects. Despite the care used during the manufacturing process, it is possible to manufacture actuators in which a substantial proportion of those manufactured must be rejected due to defects which develop during the manufacturing process.
One problem with manufacturing actuators in the foregoing manner is that actuators frequently will have one or more defective layers, i.e., layers of electrodisplacive material which will not exhibit any mechanical deformation, or reduced deformation upon the application of an electrical signal, thereby resulting in an actuator having less then the desired mechanical displacement for a given input signal. In addition, for the foregoing and other reasons, such as continuity defects in the electrode patterns, it is likely that an appreciable number of actuators having many layers of electrodisplacive material will fail to pass quality control inspections and will be required to be scrapped. The scrapping process not only results in material waste but, more importantly, in labor being expended on fabricating and testing those units which fail to pass quality control tests. The percentage of actuators which must be rejected becomes especially critical in the manufacture of deformable mirrors having large optical apertures which utilize many channels (i.e., many actuators) to provide precise control of the mirror's entire reflecting surface.
Still another problem with multiple layer actuators is that actuators must be tested to determine the amount of stroke that each actuator yields for the application of a fixed electrical signal and those actuators which do not produce a common stroke cannot be used in the same deformable mirror without the use of additional electronic controls to tailor each actuator's response so that all actuator's in the mirror exhibit a uniform response upon the application of a common signal. The need for additional electronic control reduces the reliability of the deformable mirror since the control system becomes an additional source of component failure. In addition the use of a control circuit to tailor the response of the mirror's actuators requires additional manpower to properly perform the initial adjustment of the control circuit.